1. Field of the Invention
The present invention relates to the fields of chemistry, photolithography and semiconductor fabrication. More specifically, the invention is directed to a resist composition comprising traditional resist components and additives that increase the contact angle of the resist film and lower the leaching rate of said components. The invention further relates to the resist compositions that can be used in immersion lithography without the use of an additional topcoat and a method of forming a photolithographic image, where a liquid such as water is interposed between the last lens fixture of an exposure tool and the photoresist-coated wafer.
2. Description of Related Art
The continuous drive to print smaller structures for advanced electronic device manufacturing requires the use of higher resolution optical lithography tools. Immersion lithography has the potential to extend current 193 nm argon fluoride-based technology to 45 nm critical dimensions (half-pitch DRAM) and beyond by effectively improving the depth-of-focus processing window for a given optical numerical aperture (NA). In addition, it enables lens designs with NA greater than 1.0, thus resulting in an increased resolution of optical scanners. The process requires filling the gap between the last lens element of the exposure tool and the resist-coated substrate with ultrapure water. See A. Hand, “Tricks with Water and Light: 193 nm Extension”, Semiconductor International, Vol. 27, Issue 2, February 2004.
One of the technical challenges facing liquid immersion lithography is the diffusion between the photoresist components and the immersion medium. That is, during the immersion lithographic process, the photoresist components leach into the immersion medium and the immersion medium permeates into the photoresist film. Such diffusion is detrimental to photoresist performance and might result in disastrous lens damage or contamination in a 40 million dollar lithography tool. Therefore, there is a need for a method to prevent interaction between photoresist layers and immersion fluid in an immersion lithography system.
One of the methods that have been quickly adopted by the resist community is the application of topcoat materials on top of the photoresist layer for the purpose of eliminating diffusion of materials from the photoresist layer underneath, and to prevent the permeation of the exposure medium into the photoresist film. See M. Slezak, “Exploring the needs and tradeoffs for immersion resist topcoating”, Solid State Technology, Vol. 47, Issue 7, July 2004. Since the time water was proposed as the exposure medium for 193 nm immersion lithography, several new topcoat materials have been reported.
As described above, protective topcoats are currently considered vitally important for water immersion lithography. However, this is as an additional step to conventional lithography, which adds process and material cost. Thus, there remains a need to develop resist compositions that can withstand the immersion conditions without the need for an additional topcoat.